The proposed studies focus on molecular mechanisms of signaling pathway modulation of excitability in neurons of immature hippocampus. Signaling cascades activate protein kinases, which biochemically modify substrate proteins via phosphorylation. Kinase phosphorylation of ion channel subunits is a well- characterized means of regulating channel function in neurons and therefore, membrane excitability. Despite these insights, the molecular mechanisms underlying the regulation of the membrane properties of immature neurons remain unclear. We propose that an important locus for mediating neuromodulation of excitability of neurons from immature hippocampus is through regulation of potassium (K+) channel activity by phosphorylation. This proposal focuses on Ca2+-activated K+ channels known as the small conductance SK channels. These subunits contribute to a component of the current known as the afterhyperpolarization (AHP). The AHP follows a single action potential or a series of action potentials. Therefore, the AHP plays a critical role in shaping the electrical responsiveness of hippocampal neurons beginning early in life, and modulation of this current can dramatically affect neuronal excitability. Given that K+ channels are critical to the regulation of neuronal excitability, developmental differences in K+ channel expression and kinase regulation may play a role in normal plasticity and pathological processes such as epilepsy in the immature brain. For these studies we will focus on SK2 and the current that it underlies, the apamin-sensitive AHP. The central hypothesis of this proposal is that the cAMP-dependent protein kinase (PKA) pathway regulates SK2 channel function through direct phosphorylation and that this post-translational mechanism as well as the developmental regulation of the expression of SK2 channels and the underlying mAHP contributes to the regulation of neuronal excitability in developing hippocampus. As part of our studies we will investigate the possibility that there are developmental differences in the expression and regulation of SK2 channel subunits and the apamin-sensitive AHP that may underlie the well-characterized observation that immature brain exhibits periods of increased excitability. This feature likely contributes to normal plasticity in developing CNS as well as pathology, such as increased seizure susceptibility in immature brain.